Alkaline secondary batteries, which are typified by nickel-hydrogen batteries, and non-aqueous electrolyte secondary batteries, which are typified by lithium ion batteries, are often used as a drive power source for portable electronics, such as portable telephones, including smartphones, as well as portable computers, personal digital assistants (PDA), and portable music players. Alkaline secondary batteries and non-aqueous electrolyte secondary batteries are also often used in stationary storage battery systems such as for applications for curbing fluctuations in the output of power sources for driving electric vehicles (EVs) and hybrid electric vehicles (HEVs, PHEVs), solar power generation, wind power generation, and the like, peak-shifting applications in system power for accumulating power overnight for use during the day, and so forth. Especially in EV, HEV, and PHEV applications or stationary storage battery systems, high-capacity and high-output properties are required, and thus individual batteries are increased in size and a plurality of batteries are connected in parallel or in series for use, but the use of a prismatic secondary battery is popular given its space efficiency.
When a prismatic secondary battery is used in such applications, not only must the battery capacity be increased, but also the battery must have high output; however, because a large electrical current flows to the battery in the process of high-output discharging, it is necessary to lower the internal resistance of the battery. For this reason, with the objective of reducing as much as possible the internal resistance of the battery and also of preventing fluctuations in the internal resistance, a variety of improvements have also been made as regards achieving higher reliability and lower resistance at a coupling part of the battery interior or a terminal section.
Conventionally, mechanical crimping has often been used as a method for achieving lower resistance in a terminal section of a battery or a coupling part of the battery interior. With mechanical crimping alone, however, changes in the electrical resistance over time occur when the battery is used in an EV, HEV, PHEV, or a similar environment that vibrates considerably, and therefore a boundary section of a coupling portion, created by the crimping, has been welded using a high-energy beam such as a laser, as is also disclosed in Japanese Laid-open Patent Publications Nos. 2009-087693, 2008-251411, and 2010-033766. When the entirety of the boundary section is welded, melting of the portion to which the force of crimping is applied leads to weakening of the crimping force, for which reason only a part of the boundary section is spot-welded with high-energy beams. Japanese Laid-open Patent Publications Nos. 2008-251411, and 2010-033766 disclose examples in which welding with high-energy beams was carried out along a boundary section of a coupling portion created by crimping, for each of a plurality of regions, so that a plurality of welding spots overlap with each other.
In particular, the method disclosed in Japanese Laid-open Patent Publication No. 2008-251411 for forming a connecting part that connects a collector and a terminal is described, with reference to FIG. 9, as regards a case where laser light is used as the high-energy beams.
A coupling part 60, disclosed in Japanese Laid-open Patent Publication No. 2008-251411, for a collector and a terminal is provided with: a cover plate 61 fixed to a battery outer casing (not shown); an inside insulating and sealing material 62 and an exterior insulating and sealing material 63; a collector 64 connected to a power generation element; and a rivet terminal 65. The inside insulating and sealing material 62 and the exterior insulating and sealing material 63 have a through-hole and are disposed in both inner and outer rim parts of a hole formed in the cover plate 61. The collector 64 is superposed on the inside insulating and sealing material 62. The rivet terminal 65 has a crimping part 65b that projects from a flange part 65a. 
The coupling part 60 is assembled so that the crimping part 65b of the rivet terminal 65 penetrates the outer insulating and sealing material 63, an opening of the cover lid 61, the inside insulating and sealing material 62, and a rivet terminal hole of the collector 64 from the outer circumference side of the cover plate 61; and crimping is then performed so as to cause the crimping part 65b of the rivet 65 to be pressed toward the collector 64 side, resulting in an integrated assembly. Prepared next is a work punch A that has a recess complementary to the crimping part 65b of the rivet terminal 65, and has on the rim of the recess an inclined part A1 having a predetermined angle. The work punch A is then pushed in so that the inclined part A1 abuts a distal end 65c of the crimping part 65b, and the distal end 65c of the crimping part 65b is partially deformed and shaped so that, as illustrated in FIG. 9B, the distal end 65c of the crimping part 65b becomes a circular truncated cone. The shape of the distal end 65c of the crimping part 65b is thus adjusted so as to have an obtuse angle.
Next, as illustrated in FIGS. 9B and 9C, laser spot welding is carried out by irradiating with a laser light LB from the perpendicular direction of the upper surface of the circular truncated cone of the distal end 65c of the crimping part 65b, or a direction close thereto. The range irradiated with the laser light LB at this time is a region that includes at least the collector 64 and the circular truncated cone of the distal end 65c of the crimping part 65b, and the collector 64 and the circular truncated cone of the distal end 65c of the crimping part 65b are butt-welded together. This laser spot welding offers deviation-free transmission of the energy of the irradiated laser light to both the collector 64 and the circular truncated cone of the distal end 65c of the crimping part 65b, and forms a favorable welded spot (nugget) 66 on a welded part.
Further, as illustrated in FIG. 9D, the collector 64 and the circular truncated cone of the distal end 65c of the crimping part 65b are butt-welded together so that a plurality of the welding spots 66 are formed so as to overlap with each other along the circular truncated conical portion of the distal end 65c of the crimping part 65b of the collector 64.
When the methods described in Japanese Laid-open Patent Publications Nos. 2008-251411, and 2010-033766 for forming a connecting part are adopted as a method for forming a coupling part in a battery interior, an excellent effect is demonstrated in that there is a decline in internal resistance, changes in the electrical resistance over time are less prone to occur, even in an EV, HEV, PHEV, or a similar environment that vibrates considerably, and higher reliability and lower internal resistance can be achieved in the connecting part of the battery interior or a terminal section.
However, in such a method for forming a connecting part, no consideration is given to the difference in the constituent materials of the positive electrode side and the negative electrode side, and because similar configurations are to be obtained for both the positive electrode side and the negative electrode side, mutually different problems take place on the positive electrode side and the negative electrode side. For example, in a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery, an aluminum-based metal (aluminum or aluminum alloy) is used generically as a core of a positive electrode plate, and a copper-based metal (copper or copper alloy) is used generically as a core of a negative electrode plate. For this reason, in order to curb corrosion caused by contact between dissimilar metals, a generically used positive electrode collector and positive electrode external terminal are both made of an aluminum-based metal, and a generically used negative electrode collector and negative electrode external terminal are both made of a copper-based metal.
Of these, an aluminum-based metal has low material strength, and it is difficult to ensure the coupling strength with crimping alone. For this reason, preferably, the coupling strength and the electrical conductivity of the positive electrode side are ensured by combining welding by high-energy beams and crimp-fixing. Further, because of the high material strength of copper-based metals, a firm coupling strength can be ensured with crimping alone, but, similarly with respect to the positive electrode side, it is more preferable to also carry out welding with high-energy beams. In a case where crimp fixing and welding with high-energy beams, such as is described in Japanese Laid-open Patent Publications Nos. 2008-251411, and 2010-033766, are combined, however, cracking sometimes takes place in the welded spots formed on the welded parts created by the high-energy beams, and a decline in production yield has been observed.
As a result of repeating a variety of tests aimed at pursuing the factors causing such cracking to occur in the welded spots, the present inventors have learned that one cause is that because stress is applied to a crimp fixing part, a transversely directed tensile stress is applied to the welded parts in the process of the solidification of the welded spots in the case where crimp fixing and welding by high-energy beams are combined. By further repeating experimentation to devise a configuration for the crimp fixing part, the present inventors have also discovered that it is possible to reduce the tensile stress in the lateral direction for the welded spot in the process of the solidification of the welded spot, and that it is possible for cracking to be less prone to take place at the welded spot, thus completing the present invention.